Natural gas (NG) is becoming an increasingly abundant resource in the US and around the world.1 While NG is used for heating it would be ideal to upgrade this resource to chemicals and liquid fuels. This could augment or potentially replace petroleum as the feedstock for chemicals and fuels. Natural gas is also abundantly available in remote locations where transportation to centers of use is not economically viable. In these cases it would be desirable to have an inexpensive process to convert the natural gas to a more easily transported form such as a liquid. However, the existing high-temperature, indirect processes based on the conversion of natural gas to syngas (CO/H2) followed by conversion of the syngas to chemicals and liquid fuels are too energy and capital intensive to economically compete with products from petroleum. Current processes for the conversion of natural gas to fuels and chemicals require high-temperature (>800° C.) to generate synthesis gas or olefins. These processes are very capital and energy intensive and are only economical at very large scales. Therefore, there is a need for economical and environmentally benign processes for production of lower alcohols and other oxygenates from natural gas alkanes. It is generally considered that a direct, lower temperature (<300° C.), selective process to convert the gases in NG (primarily methane, ethane and propane2) to liquid products such as alcohols could be used to generate chemicals and liquid fuels at much lower cost than the existing high-temperature, indirect syngas processes.
A technology for the direct low-cost conversion of the major components of natural gas (methane, ethane, propane) to liquid fuels and chemicals such as oxygenates would provide a path to increased value for these sources of natural gas. The potential market for such technology is large; e.g, the global market value for ethylene glycol is over $20 billion/yr with the US at over $4 billion/yr. The markets for other oxygenates such as methanol, ethanol (that can also be inexpensively converted to ethylene), isopropanol, propylene glycol, etc., are also very large. The liquid fuels market is enormous; a 2% penetration of the projected US transportation fuels market, equivalent to the projected annual growth rate in the US, would represent about 50 plants of 14,500 barrel per day capacity.
An important approach that has emerged in the last few decades is the design of molecular (homogeneous) catalysts for the oxidative functionalization of alkanes based on the CH activation reaction. This involves reaction of an M-X catalyst with a hydrocarbon CH bond (R—H) under relatively mild conditions to selectively generate a M-R intermediate that can be converted to the desired R—X product with regeneration of M-X (eq.1).

There has been significant effort in this area of research with homogeneous3-23 as well as heterogeneous catalysts24-26 and substantial progress has been made in recent years. Most of the work on the homogeneous systems have been primarily based on transition metals (with unfilled d-shells, d<10) such as Pt,3,4,16 Pd,14,17-19,23 Rh,20-22 and Ir.7-10 In contrast, relatively few studies have been directed toward the classic main group elements with filled a d-shell (d10.
In 1993, we reported an example of a main group, metal cation, HgII, in the superacid solvents, concentrated H2SO4 and CF3SO3H, for direct conversion of methane to methanol esters.15 In spite of the simplicity of the HgII system, it was not further developed due to lack of reaction in more practical weaker acid media such as CF3CO2H (TFAH or HTFA), CH3CO2H (HOAc), or aqueous acids where product separation could be practical. Another key issue was that the reactions of ethane and propane were unselective with the HgII system. We originally proposed an electrophilic CH activation mechanism for the HgII system. However, later work by Sen based on the observation of products resulting from C—C cleavage reactions with higher alkanes suggested that Hg(II) in superacid media was sufficiently oxidizing to initiate free radical reactions.
This possibility for unselective radical reactions with higher alkanes was also considered by Moiseev and coworkers in the early 1990s in the reaction of methane, ethane, and propane in TFAH with several strongly oxidizing salts that were known to be effective for oxidizing hydrocarbons by free radical mechanisms.27,28 The initial report showed high yield and selectivity for the stoichiometric reactions of CoIII with methane to Me-TFA. Carrying out the reaction with CoII in the presence of O2 showed a TON of ˜4 for Me-TFA along with comparable amounts of CO2 generated by solvent decarboxylation. Along with CoIV peroxo species both radical and non-radical mechanisms were considered. In a later report focusing on the reactions of ethane and propane the authors proposed a free radical mechanism to account for the extensive C—C cleavage and over-oxidation reactions with these higher alkanes.
In our own work on HgII the main group d10 cation, TlIII was found to be active for methane oxidation to the ester. However, this was only examined in superacid media and only with methane. As with HgII this system was likewise inactive in more practical weaker acids. No further work was carried out on the TlIII system in TFAH or with higher alkanes and PbIV was not examined. The primary basis for this was the recognition that both TlIII (Eo=1.2 V) and PbIV (Eo=1.5 V) are stronger oxidants than HgII (Eo=0.9V).29 Consequently, on the basis of the general considerations at that time, we considered that these main group cations would be more likely than HgII to initiate unselective radical reaction with the higher alkanes. To our knowledge, since those early publications in 1990's there are no reported, deliberate studies of reactions of higher alkanes with those or other strongly oxidizing main group electrophiles.